1. Field of the Invention
The present invention relates to a closed loop transmission power control method in a CDMA (Code Division Multiple Access) communication system for controlling transmission power of a transmitter such that an Eb/IO value of a signal received at a receiver from each transmitter is equal to a reference value, and more particularly to a method of updating a reference value which serves as a reference in a closed loop transmission power control.
2. Description of the Related Art
In recent years, a CDMA communication method draws attention as a communication method for use in a mobile communication system due to its resistance to interference or disturbance. In the CDMA communication method, for maximizing its communication capacity, it is necessary to adjust transmission power of each of a plurality of transmitters such that input signals from the transmitters have the same level at an antenna end of a receiver. To that end, in a portable phone system of a cellular scheme, for example, a check is made to determine whether the ratio of received signal power to interference power recognized at a receiver in a base station is higher or lower than a preset expected value. Based on the determination result, a signal for requesting an increase or decrease in the transmission power of a mobile station is inserted into a transmitted signal from the base station to the mobile station. The mobile station, upon receiving the signal, controls the transmission power in accordance with the signal inserted into the transmitted signal, thereby achieving control for obtaining a maximum channel capacity. Such control of the transmission power of the mobile station is referred to as high speed closed loop control since it is performed at a relatively high speed.
Typically, since the quality of communication is ultimately determined by a bit error rate (BER), it is essentially preferable that a high speed closed loop is controlled based on the BER. However, the high speed closed loop control requires the loop to converge quickly as mentioned above, while the measurement of the BER usually takes a long time. For this reason, the high speed closed loop control employs a reference which can be quickly recognized instead of the BER. As such a reference, a signal power to interference power ratio (hereinafter referred to as xe2x80x9cEb/IOxe2x80x9d) is used, for example.
The Eb/IO is usually proportional to the BER. However, the relationship between the Eb/IO and BER may be changed depending on conditions of a communication path, and a minimum level of a required BER may not be ensured even when an Eb/IO value serving as a reference is obtained. To address this problem, outer loop control is used for updating the value of the Eb/IO serving as a reference value. In prior art outer loop control, the BER is measured for a certain period of time and the value of the Eb/IO serving as a reference value is updated in accordance with the obtained BER.
However, the measurement of the BER to 10xe2x88x923, for example, takes a minimum of one second at a data speed of approximately 10 k bit/S. The measurement of the BER to 10xe2x88x926, in turn, takes as much as 1,000 seconds at the minimum. Additionally, the measurement of the BER requires the use of known data on the reception side and transmission side. However, since typical data is not known, known data used for maintaining synchronization, for example, must be used for the measurement. Such data for maintaining synchronization is redundant information in data communication and usually has a small amount of data transmitted. As a result, the measurement of the aforementioned BER using such known data used for maintaining synchronization takes a significantly long time. Therefore, in the outer loop control which updates a reference value in closed loop transmission power control in accordance with the measured BER, a longer update period of an outer loop prevents a response to a quick change in a communication path, resulting in the possibility of degradation of communication quality.
It is an object of the present invention to provide a method of updating a reference value in high speed closed loop transmission power control which can respond to a quick change in a communication path.
To achieve the aforementioned object, in the present invention, a path with a smallest accumulated value of distances in codes from a received sequence is first determined as a decoded sequence out of respective paths on a trellis diagram in a Viterbi algorithm. Next, a reference value is updated based on a likelihood which is an accumulated value of distances in codes between a decoded sequence and a received sequence for a certain period of time.
In this manner, the present invention performs update of a reference value in a high speed closed loop based on a likelihood obtained when convolutional-coded signals are decoded with the Viterbi algorithm, thereby making it possible to shorten an update period of a reference value as compared with the update of a reference value based on the BER, and to respond to a quick change in a communication path.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.